In continuous casting processes, the use of gases injected down the stopper has been shown to have significant benefits on the quality of metal being cast. For example, inert gases such as argon or nitrogen can be injected to reduce the problems due to alumina build-up and clogging or to assist in removing solidification products from the vicinity of the discharge nozzle. Reactive gases may also be employed when the melt composition needs modifying. Conventionally, the stopper is provided with an internal chamber connected to gas supply means on the one end and to a gas discharge port at the other end.
Various systems have been developed to ensure an accurate measured flow of gas is supplied to the stopper. Problems have been encountered with sealing such systems and ensuring that the gas follows its intended path and is not wasted. Stoppers which have proved to be successful in meeting many of these problems are disclosed in EP-A2-358,535, WO-A1-00/30785 and WO-A1-00/30786.
However, even given such valuable improvements, there is a need to address other problems. One such problem is apparent due to the effect during pouring of large volume of melt of metal flowing past the nose of the stopper through the discharge nozzle. A negative pressure can be generated at the stopper tip which can be transmitted through the gas discharge port into the body and back to the supply pipework where it may exploit any inadequate joints causing air suction into the gas stream with significant detriment to the quality of the metal being cast.
Various solutions have been proposed to eliminate this risk which involve restricting the gas flow within the stopper thereby seeking to create a positive pressure within the stopper. For example, a simple restriction between the internal chamber and the gas-discharge port to provide control is known. At the required pressure, the orifice size of the internal chamber was calculated to be between 0.2–0.5 mm in diameter and, as such, is extremely sensitive to blockage by debris or dust carried in the gas stream, thereby causing loss of flow. It is also known to insert a gas permeable plug into the stopper to provide the required restriction to flow and to pressurise the stopper. However, these systems suffer from the problem of changes in the permeable metal comprising a stopper body having an internal chamber and a gas discharge port, a bore connecting the internal chamber to the gas discharge port, calibrating means being provided in the bore to provide a restricted path. The calibrating means are formed by using a sacrificial void former to form a portion of the bore connecting the internal chamber to the gas discharge port thereby providing a restricted slit-like form path which is said to offer a predetermined resistance to flow and tends to maintain a positive pressure within the stopper. However, the formation of a slit-like path made by using a sacrificial void former is extremely unreliable and does not allow the formation of a restriction with a precise predetermined resistance to flow. Further, this formation method does not allow the formation of very narrow passages. It is to be understood that a positive pressure within the stopper means that the pressure is at least equal to the pressure outside the stopper.
According to another known system disclosed for example in FR-A-2,787,045, there is provided a mono-block stopper adapted to deliver gas during pouring of molten metal comprising a stopper body having an internal chamber and a gas discharge port, a bore connecting the internal chamber to the gas discharge port. Calibrating means are provided under the form of a Venturi-tuyere inserted into the internal chamber. Such a design of the calibrating means does not permit flexibility in the manufacturing process. Further, special precautions must be take to avoid the problem of clogging of the Venturi-tuyere for example by dust.
The present invention aims to overcome or at least mitigate the above problems associated with the prior art stoppers and, in particular, their lack of reliability.
According to one aspect, the present invention concerns thus a mono-block stopper adapted to deliver gas during pouring of molten metal comprising a stopper body having an internal chamber and a gas discharge port, a bore connecting the internal chamber to the gas discharge port, calibrating means being provided in the bore to provide a restricted path. This stopper is characterised by the fact that the calibrating means comprise a rod having at least one axially-extending gas passage therealong, the gas passage having a section such as to offer a predetermined resistance to flow.
The predetermined resistance to flow of the gas passages extending along the rod is calculated to permit a very precise and reliable control of the relationship gas-flow/internal pressure and/or to maintain a positive gas pressure within the stopper.
The use of such a rod which can be inserted into the stopper body at the very end of the manufacturing process of the stopper permits an extreme flexibility in the setting up of the “predetermined” resistance to flow so that the stopper of the invention can be adapted to a wide range of operational parameters simply by changing the rod. Furthermore, the rod—being manufactured separately—can received much more attention than if made together with the stopper and is therefore much more reliable. Such rods are available commercially for use as thermocouple sheaths.
Preferably, the rod is made from a gas-impermeable refractory material so that gas leaks at the level of the rod are avoided, thereby increasing the reliability of the calibration. Advantageously, the material is also wear-resistant so that the predetermined resistance to flow remains constant during the entire life of the rod. Suitable materials include mullite, a fired alumino-silicate, alumina re-crystallised alumina, zirconia-alumina and other high-refractory materials having the required properties.